1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and, more particularly to an image forming apparatus and an image forming method for printing color data using plural color components including black.
2. Description of the Related Art
In image forming apparatuses such as a digital color multi-function peripheral (hereinafter referred to as digital color MFP) and a color printer, a form of performing printing (image formation) using a color component of black (K) in addition to plural color components of yellow (Y), magenta (M), and cyan (C) is often used.
In the image forming apparatuses of this form, a method of determining presence or absence of achromatic color data included in color data read by a scanner unit of the digital color MFP and color data inputted to the color printer from a personal computer or the like and printing an area, which is determined to be of achromatic color data, with a single color of black (K) rather than a combination of the plural colors is often used.
On the other hand, concerning color data that is not an achromatic color (i.e., chromatic color data), first color data (e.g., red (R), green (G), and blue (B)) is replaced with second color data (e.g., yellow (Y), magenta (M), cyan (C), and black (K)).
In this color conversion processing, the color conversion processing is performed according to a method corresponding to a printing object such as a method of preserving brightness (i.e., a method of prioritizing reproducibility of brightness) or a method of emphasizing chroma and contrast.
In the determination of presence or absence of achromatic color data (hereinafter referred to achromatic color determination), usually, determination using a predetermined threshold is performed. Therefore, data determined as achromatic color data includes not only true achromatic colors but also chromatic colors extremely close to the achromatic colors (hereinafter referred to as semi-achromatic colors).
Conventionally, when achromatic color or semi-achromatic color data is converted into a single color of black (K) (this conversion is hereinafter referred to as black conversion processing), only the method of preserving brightness is used.
Therefore, when color conversion is performed using the method of preserving brightness as the color conversion processing, for example, it is possible to realize continuous smooth brightness change for a gradation image that continuously changes from a chromatic color to an achromatic color. However, when color conversion is performed using the method of emphasizing chroma and contrast as the color conversion processing, preserving of brightness is not guaranteed. Thus, there is a problem in that discontinuity of brightness (hereinafter sometimes referred to as “brightness discontinuity”) occurs in a boundary of an area subjected to the color conversion processing and an area subjected to black conversion processing.